US2801805A

US2801805A - Method

Info

Publication number: US2801805A
Authority: US
Publication date: 1957-08-06
Anticipated expiration: 1974-08-06

Description

Aug. 6, 1957 c. l. JOHNSON v 2,801,305

METHOD AND APPARATUS FOR WINDING POTENTIOMETERS Filed May 25,1955

3 Sheets-Sheet 1 Aug. 6, 1957 c. l. JOHNSON METHOD AND APPARATUS FOR WINDING POTENTIOMETERS 3 Sheets-Sheet 2 Filed May 25, 1953 mu oHrmV 0.0 $0 00? SWN METHOD AND APPARATUS FOR WINDING PO'IIENTIOMETERS Filed May 25, 1955 Aug. 6, .1957 c. l. JOHNSON 5 Sheets-Sheet 3 RN w M N w mJ T R U c United States Patent Ofiice METHOD AND APPARATUS FOR WINDING POTENTIOMETERS Curt L Johnson, Vestal, N. Y., assignor to International Business Machines Corporation, New York, N. Y., a corporation of New York Application May 25, 1953, Serial No. 357,165 13 Claims. (01. 242-9 The present invention relates to winding apparatus and more particularly to a method and apparatus for winding potentiometers and to an improved potentiometer produced by said apparatus.

. An object of this invention is to provide an improved winding apparatus.

Another object of the invention is to provide an improved method for winding potentiometers and the like.

Another object is to furnish an improved potentiorneter apparatus.

Still another object of the present invention is to furnish an improved winding apparatus for automatically producing potentiometers having characteristics indentical' to a reference potentiometer.

- A further object of this invention is to provide an improved method and apparatus for producing precision potentiometers having precise linear or nonlinear characteristics.

A still further object of the invention is to provide a potentiometer winding apparatus for winding a resistance element in the form of a series of coils on a winding form wherein control means are furnished for automatically determining the rate at which the coils are wound.

Another object is to provide a potentiometer apparatus in which the spacing of the coils of the resistance element is variable depending upon the resistance of the wire making up the -coil.

(Still another object of the instant invention is to provide a potentiometer apparatus of the type having a resistance element in the form of a helical spring, said spring being wound on a core in the form of a helix, the spacing of the coils being a function of the resistance thereof, the spacing being maintained by a thermosetting resin connecting the portion of the coils engaging the core to the core.

, Other objects of ,the invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of examples, the principle of the invention and the best mode, which has been contemplated, of applying that principle.

In the drawings:

Fig. l is an electromechanical schematic diagram of the winding apparatus;

' Fig. 2 is an electrical schematic diagram of the winding apparatus;

Fig. 3 is a schematic diagram of a typical servoamplifier which may be used with the winding apparatus;

'Fig. 4 shows an enlarged section of a number of the coils making up the resistance element and the contact means for engaging the coils;

-' Fig. 5 is an elevation of the potentiometer in a finished state; and

Fig. 6 is a view taken on line 6-6 of Fig. 5, parts of the view being broken away for greater detail. I Similar reference characters represent similar parts throughout the several views.

The manufacture of high precision otentiometers such as those used in modern analog computers is extremely diflicult, particularly where the tolerances are quite low. These potentiometers are usually of the multiple-turn helical type and often have an overall linearity requirement of less than .02 percent.

At present, one method of manufacturing precision potentiometers is to accurately wind the resistance wire on a mandrel in the amount desired. The spacing of the wire on the mandrel may be accomplished through lathe feeding means. The mandrel is then wound around the potentiometer core to form a helical coil. A wiper arm may then ride in a groove adjacent the coil and pick off various levels of potential between the two ends of th coil.

Difficulties arise in this particular method due to differences in the wire diameter, the coil diameter, the coil spacing, and the non-uniformity of resistance of the wire itself.

The present invention overcomes these difiiculties as will be more apparent hereinafter. In order to accomplish uniform resistance and in some measure take advantage of variations in wire diameters, wire resistance, and coil diameters, it is proposed to wind the resistance wire in the manner of a spring. By Winding a spring of a suitable resistance material with a spring index of approximately six, and with sufiicient initial tension so that the spring is closely wound, the spacing of each coil is directly proportional to the mean diameter of the spring and inversely proportional to the wire diameter. It will be seen that the resistivity of the wound spring is a function of similar relationships.

The spring is formed by winding the wire on a mandrel into coils closely adjacent each other. The angle of feed of the wire to the mandrel is adjusted so that it forms an acute angle with respect to the portion of the spring already wound. Thus, an initial tension can be placed in the spring. The mandrel is removed and the spring is ready for use as the potentiometer resistance element.

By placing the length of spring having a desired overall resistance on a suitable bobbin, a potentiometer of single or multiple turns can be wound on a winding form from the bobbin. The bobbin and the form to be wound are both driven and means are provided to vary the speed of the bobbin to permit a uniform resistance element to be wound on the form.

For a more detailed description of the invention reference is made to Fig. 1, which is an electromechanical schematic diagram of the wind-ing apparatus, and Fig. 2, which is an electrical schematic of the circuitry involved. In the drawings, electrical wires are indicated by solid lines and mechanical shaft connections are indicated by dotted lines. After the resistance wire has been wound on the mandrel so that the coils are in contact with each other the mandrel is removed. Sufficient initial tension is used in the initial winding to assure accurate coil spacing. The length of coils appears as a helical spring.

For a more detailed description of the manner of winding the resistance Wire on the mandrel reference is made to application Serial No. 165,623, filed June 2, 1950, now Patent No. 2,650,638, dated September 1, 1953, for Spring Coiling Device by Curt I. Johnson and James A. Nord.

A length of straight wire is placed adjacent the length of coils. One end of the resistance element and one end of the straight wire are secured to one end of the storage bobbin. resistance element is expanded to separate adjacent coils thereof along the length of the straight wire. When sufficient spacing is provided so as to prevent shorting between the coils the coils are wound on the bobbin.

The storage bobbin includes a cylindrical surface which Patented Aug. 6, 1957' The straight Wire is then made taut and the 3 is provided with a helical groove extending the length thereof. The resistance element is adapted tofit within the groove. The storage bobbin is shown schematically in Fig. 1 at 10. The end ofthe resistance element fixed to the bobbin is electrically connected to slip ring' 11 associated. with the bobbin: The otherend of the resistance element is connected to one end of" the potentiometer form or core illu-stratedaby the reference numeral12, the detailsof' which are shown in Figs: and 6. The slip ring 13 is electrically connected to the last named end of. the resistance element. A DwC'. potential is applied across. the entire spring by connecting to slip rings 11- and 13.. Once the spring resistanceelement is connected to: the potentiometer core, the straight wire may be removedfrom the bobbin.

Areference voltage potential is obtained by providing a; master potentiometer illustrated by numeral 1 4: This master potentiometer is preferably a potentiometer with the best possible degree of precision. It will be under stood. that the other means could be provided as long as a precision source of voltage isfurnished to which the. potentiometer being. wound can be compared.

A drive motor 15 is excited by a 115 volt 400 C. P. S; A.-C. power supply and is shaft connected through gear box 16 to'a slip ring 17 associated withpotentiometer 14-; schematically shown, slip ring 1 7 is secured to an arm 18 which. is adapted. to move around the periphery of the potentiometer. A contact supporting member 19 is slidably mounted. on arm 18 and is adaptedto followa helical groove, adjacent the helical. resistance: winding of the potentiometer. A contact 20 is mounted on member 19 andis adapted to ride'on the potentiometer resistance element. The ends of. this last recited resistance element are connected .to. a.D.-C. power supply so that a potential isplaced thereacross;

Slip ring 17 is shaft connected to slip ring 13 and core 12. Both. slip rings are connected to one: side of. a gear differential 21. Another side of the dilferential is shaft connected to slip ring 11 and bobbin 10. A motor. generator servo unit 22 is shaft connected to the third; side of the differential andis adapted to vary the drive speed of the bobbin with respect to the: core 12 in accordance with the rate at which the spring must be wound on the core to reproduce the master potentiometer. That is, the tension of the spring can be varied in order to control the spacing of the coils which are wound-on the core.

In order to control the motor generator servo unit 22 a contact 23 is mounted on a contact supporting member 24, said member being sl-idably mounted. on a fixed arm 25 adjacent core 12. A guide groove is provided ad jacent the helical groove of the core for receiving a finger 26, shown in detail in Figs. 5 and 6, saidfinge-r being: secured to member 24. As the core is rotated, finger 26' rides in the guide groove and causes member 24 to move along arm 25. gaged with the spring resistance element which is being. wound on the core- The engagement is made just as the spring resistance element is being laid in place on the core.

Reference is now made to Fig. 2 along with Fig; l. The spring resistance element being wound on core 12 is illustrated by numeral 27. The DC. power supply places the same potential across resistance elements 14' and 27'. Contact 20, which picks off a potential from. potentiometer 1-4, is connected to ground potential. Terminal 28 of chopper 29 is also grounded. Arm 23 picks off a potential from resistance element 27. and supplies this potential to terminal 36 of the aforementioned chopper.

Chopper 29 further includes an arm 31, said arm being pivoted at one end and adapted to contact terminals 28 or 30 at the other end thereof. A series connected coil 32 and capacitor 33 are connected to an A.-C. power supply of 1 15 v. 400 C. P. S. Therefore, arm 31 is moved In this manner contact 23 is alwaysv on- 4, I into contact alternately with terminals 28 and 30 at the rate-of-400cyclesper second. r

In view of the above it will be seen that a square wave output signal will be produced as long as there is a potential difference between terminals 28 and 30. This output signal is RC coupled by resistor 34 and capacitor 35 to transformer 36 the output of which is applied across a resistor 37. An output signal. is picked off the resistor at a desired level and fed to a first amplifier 38 where the signal is amplified. The amplifier output signal is fed to a phase adjusting network 39 from which it is fed to servoamplifier 40.

Amplifier 40 may be in the form of a conventional servoamplifier of a number of stages, sufficientto obtain proper amplification. While the details of the amplifier do not form a part ofthe inventiona schematic circuit of an amplifier which may be used is shown in Fig. 3. Briefly, the amplifier includes a voltage amplifying input stage 41', the output of" which feeds across a transientbypass capacitor 42 and is RC coupled to asecond voltage amplifying stage 43. The output voltage from stage 43' is fed to a phase shifting network 44-which' advances the phase of the signal by The output signal from network 44 is supplied to one side of a duo-triode 45-, the output of which is supplied through a voltage divider network 46 to the other side of the duo-triode. Thus, tube 45. acts as a push-pull amplifier stage. The output signals from tube 45 are out of phase and are RC coupled to a second duo-triode 47' which acts as a pushpull power stage of amplification. From the plates of tube 47 the output is limited by tube 58 and coupled through transformer 48 to the control winding 49 of motor 50, said. motor being a portion of servo unit 22. Motor 50 is shaft connected to the generator portion 51 of said unit. Both the motor and generator are of the two-phase type. A reference potential of 26 v. 400 C. P. S., A. C. isapplied to the fixed windings 52 and 53- from the system power supply. The output winding- 54 of the generator feeds back to the amplifier degenerativelyfor damping purposes. As shown in Fig. 3 the generator output feeds to the control grid oftube 43- t-hrough are sister 55.. The phase difference between the feed-back voltage from the generator and theinput' voltage supplied from amplifier 41 is 180.

The shaft output from servo unit 22 is-connected to differential 21, as shown in Fig. 1, so. as to control the rate of rotation of storage bobbin 10.

The overall mode of operation of the present invention will now be described. A D. C. potential is applied across themaster potentiometer 14 and the resistance element 27- which is being wound on thecore 1 2- from storage bobbin 10. A drive motor 15 isconnect'ed' to drive the master potentiometer 14 and the core 12. Motor 15 drives storage bobbin 10 through differential 21. As the resistance element is being wound on the core a comparison is made between the potentials at

contacts

20 and 23. If there is no difference in the potentials no output signal is fed through the chopper and amplifiers to servo unit 22. The shaft connecting the servo unit to the differential is-stationary and therefore storage bobbin 10 and. core 12. are-rotated at the same speed. However, if there. is a potential difference between contacts 20 and'23 the difference potential exists between terminals 28 and 30'. It will be seen that terminal 28 is placed at ground potential. It terminal 3! is above ground potential the output voltage from, the chopper will be of one phase and if terminal; 30 is below ground potential the output voltage from the chopper will be of the opposite phase. The phase of the output signal determines the direction of rotation of the shaft output from servo unit 22 and the amplitude of the output signal determines the rate of shaft rotation-.. Therefore, if too much of the resistance element 27 has been wound on the core at a particular instant the shaft output from. the servo unit,v through the differential, decreases the rate of rotation of bobbin 10. If not enough. off'the.

resistance element 27 has been applied to the core the servo unit shaft output is in the opposite direction and increases the rate of rotation of bobbin 10. The increase or decrease in the rate of rotation of bobbin decreases or increases, respectively, the tension on the spring resistance element. The tension, in turn, controls the spacing of the coils andtherefore the number of coils which can be applied to the core with respect to time.

Fig. 4 shows in a somewhat exaggerated manner the space relationship between coils of the resistance element 27 which are constructed from wire of varying diameter. For example, the wire of coils 60, 61 and 62 is smaller in diameter than the wire of coils 63, 64 and 65. It will be noted, however, that the spacing between coils 60, 61 and 62 is greater than that between coils 63, and 65. The resistance of the individual coils decreases with increase in wire diameter and the variable spacing compensates therefor.

In order that the correct spacing of the coils is always maintained provision is made for securing the coils to the winding form. One way which has shown good results is to mix an epoxy resin into a liquid state and apply a thin film thereof to the helical groove into which the resistance element is to be placed. This film is allowed to air dry until it becomes tacky, which occurs toward the end of the pot life of the resin at this time the winding operation is then performed. After the element is completely wound on the core, the complete unit is placed into an oven of 60-80 C. and slowly rotated. The resin becomes soft and then hardens to a solid state. The hardened substance covers the lower portion of the coil to impart additional rigidity thereto.

After the core has been completely wound it may then be assembled with the remainder of the potentiometer. The complete assembly is shown in Figs. 5 and 6. The winding form or core 12 is fixedly mounted on the base plate 66. The, mechanicalconnection of the ends of the resistance element 27 is illustrated by

numerals

67 and 68, said connections being at opposite ends of the core and spaced 180 apart. The electrical connections for the potentiometer are spaced 90 from the mechanical connections. As shown, a small Wire 69 is led from a connection at one end of the effective resistance via a longitudinally extendingslot 70 to a terminal post 71. A heavier wire 72 is connected to the terminal post for connection to one side of the power supply which may be either an A. C. or D. C. source. A small wire 73 connects the other end of the effective resistance and the terminal post 74. A heavier wire 75 is connected to the last named post and extends through a longitudinally extending aperture 76 to therother side of the aforementioned power supply.

The contact supportingarm 77 is connected to one end of a member 79, said-last named member being secured centrally thereof to a shaft 80. Shaft 80 extends through core 12 and base plate 66 and is journaled therein. A second arm 78 is connected to the other end of member 79 and is adapted to counterbalance arm 77. The contact 23 and its supporting member 24 are similar to those shown in Fig. 1 and are illustrated by similar reference numerals. The finger 26 is adapted to ride in the helical groove 81.

The electrical pick-off is through contact 23, supporting member 24, arm 77 and member 79 to a slip ring 82. Slip ring 82 is secured to shaft 80 along with member 79. A pair of wire contacts 83 are mounted on a terminal plate 84 and are adapted to engage slip ring 82 for picking off the electrical potential thereon. A wire lead 85 is connected to terminal plate 84 and fed through aperture 76 to the other end of the core. A slot 86 is provided in base plate 66 to permit leads 72, 75 and 85 to be led from the potentiometer.

From the above it will be seen that a potential is applied to the coils between the connection of

wires

71 and 73. Rotation of arm 77 permits contact 23 to pick off any potential between the points of applied po tential.

The above detailed description relates to a preferred embodiment of the invention. It will be understood, however, that various elements are shown which serve only by Way of example as a means for carrying out the invention. The master potentiometer could be replaced by any kind of voltage supply which would supply a potential with which the potentiometer being wound is to bear a relationship. For example, it will be seen that a switchable decade box could be used. A source of potential could be applied across a resistor or a series of resistors, there being relay controlled means programmed to switch in and out small increments of resistance to provide a very accurate reference potential. The amount of initial tension which is placed in the spring resistance element may be increased or decreased in accordance with the requirements of the situation. While a chopper has been shown as a means for obtaining a usable output signal from the voltage difference between the potentiometer pick-off, it will be seen that other means could be provided without departing from the scope of the invention. It is but necessary that a signal be obtained which is a measure of the potential difference of one with respect to the other. It should be further understood that potentiometers other than linear potentiometers may be manufactured with the present apparatus. By providing a particular non-linear potentiometer or other voltage source as a reference the servomotor control will regulate the rate of feed of the resistance spring to the core. The only limitation comes in the amount the individual coils can be spaced. That is, they cannot be so close together as to provide a short circuit or so fart apart as to prevent engagement between at least one of the coils and the contact means. Too great a spacing results in an open circuit.

The advantages to be gained from the present invention are many. It is not necessary to wind the wire resistance material on the mandrel in coils having a definite space therebetween. This simplifies matters considerably since the precision required in winding the material on the mandrel is lessened. The overall resistance of the potentiometer may be accurately set prior to placing the element on the core which is not the case in producing a potentiometer with a resistance element wound on a mandrel. Where a mandrel is used a certain length is required to obtain the correct number of turns, which, in most cases is ten. Therefore, in winding the wire on the mandrel variations in wire diameter and the resistance of the wire itself cause more or less wire to complete the mandrel which means more or less resistance than an amount which may be specified. For this reason trimming resistors must be added to the circuit in which the potentiometer is to be used.

In the conventional potentiometer having a mandrel there is considerable difiiculty experienced by capacitance between the coils and the mandrel which is usually at ground potential. Capacity effects between the individual coils and the mandrel are in parallel so that they cause phase shift where A. C. potentials are applied. In the instant invention the capacity effect is from coil to coil so that the capacity effects are in series. Thus, there is very little effect on the potentiometer.

The present invention produces a potentiometer, the linearity of which is little affected by changes in the diameter of the resistance wire and changes in the re sistance of the wire itself along the length thereof. For example, should the wire diameter increase slightly for a number of coils the resistance of these coils will be less than the resistance of a similar number of coils of a lesser diameter. However, when the spring resistance element is placed under tension, the coils of the larger diameter will be spaced apart less than the coils of the smaller diameter. At the same time, more coils of the larger diameter wire will be placed on the potentiometer core than of the smaller diameter wire, during a pre- ,7 determined period of time. Thus, a self compensating feature is obtained. A further advantage is obtained in. checking the potential adjacent the position where the coils-are placed on the core. This enables compensation for any variation from. a predetermined potential before further coils are. wound on the core. That is, all tolerances. or variations in the coils are checked at the time of Winding.

Since each. individual coil is bonded to the winding form there is no opportunity for looseness of coils or shifting of coils which often occurs on the mandrel type potentiometer. This eliminates electrical noise to a considerable degree.

While there have been shown, and described and pointed. out. the fundamental novel features of the invention as applied to a preferred embodiment, it will be understoodthat various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1., A method of manufacturing electrical apparatus, the steps comprising forming a length of wire into a helix to produce a resistance element, feeding the thus formed resistance element in its helical. condition on to a supporting device, and regulating the rate of feeding said helix to said. supporting device.

2.. An apparatus for producing resistance devices, a resistance element in the form of a series of successive coils, a winding form, said resistance element being secured "to said form, means for rotating said form, means for feeding the remainder of said resistance element to said form with the portion of the resistance element between said form and the feeding means still comprising a series of successive coils, and means for controlling the rate of feed of said coils to said form.

3. A winding device comprising a resistance element in: the form of a series of successive coils, a winding form, means for feeding said resistance element while it is in the form of successive coils to said form, means for placing a potential across said resistance element, pickotf means adjacent the point of entry of the coils of said resistance element on said form, a variable potential source, and means responsive to the variation of the potential at said pick-oif means from the potential from said variable potential source for controlling the resistance per unit length of the portion of the resistance element being applied to said form.

4. A device for winding potentiometers, a resistance element in the form of a series of successive coils, a form, a storage device having one end of said resistance element secured thereto, the other end of said resistance element being secured to said form, drive means for rotating said form to wind said resistance element thereon from said storage device, contact means engaging said resistance element adjacent the point of application thereof to said form, means for applying an electrical potential across said resistance element adjacent the ends thereof, meansv for comparing the potential at said contact with a potential which varies during the winding operation according to a prescribed characteristic, means for produeing an output signal in accordance with the diiference between the potential at said contact and said potential which varies, and means responsive to said output signal for controlling the tension of said resistance element being fed to the form.

5. A method of manufacturing electrical apparatus comprising the steps of forming a length of resistance wire into a series of sub-portions such that the spacing of adjacent snb-portions may be varied, winding the thus formed wire on a form, and regulating the spacing of adjacent sub-portions as they are being applied to said form;

6. A method ofv manufacturing electrical apparatus comprising the steps of forming a plurality of coils in a length of' resistance wi-re so that the spacing of adjacent coils may be varied, winding the thus formed wire on a form and regulating the spacing of' adjacent coils during thetime theyare being applied to said form.

7. A method of manufacturing electrical apparatus comprising the steps of' forming a plurality of coils in a length of resistance wire by winding said wire on a mandrel, removing said coils from said mandrel so that there remains a springlike resistance element, winding the springlike resistance element onto a form, and regulating the spacing of adjacent coils as the coils of the resistance element are being applied to said form.

8. Ina method of manufacturing electrical apparatus, the steps comprising forming a resilient resistance element in the form of a series of coils whose spacing may be varied from a length of resistance wire, separating adjacent coils so that there is a space therebetween, applying a. substantially constant potential across a portion of said resistance element so that there is a substantially constant current flow tlierethrough, winding said element on a form, comparing the potential at each coil of said element adjacent its point of entry on said form with a source of potential which varies in accordance with a desired function, and spacing the coils being applied to said form as a. function of the difference between the two potentials compared.

9'. A method ofmanufact'uring electrical apparatus, the steps comprising forming a resistance element from a length of resistance wire such that the resistance per unit length of said elementmay be varied after said element is formed, winding the thus formed resistance element on, a form, and controlling the resistance per unit length of said resistance element as said element is wound on said form.

10. A device for winding resistance apparatus, a resistance element in the form ofa series ofcoils, a winding form having one end of said resistance element connected thereto, means for rotating said form so as to wind said resistance element thereon, a source of potential connected acrosssaid' resistance element including the portion already wound onv said form and the portion yet to be wound, pick-01f means for obtaining the potentials on said element adjacent the point of entry of said element on said form, means for supplying a reference potential which follows a' predetermined characteristic during the course of winding said element on said form, means for comparing the potential at said pick-0E means with said reference potential to providean error signal, and means for controlling the spacing of adjacent coils of said element' as they are applied to said form in accordance with said error signal.

11. A device for winding resistance apparatus, a resistance element in the form of a series of coils, a winding form having one end of said resistance element connected thereto, means for rotating said form so as to wind said resistance element thereon, a master potentiometer including a slider which traverses a resistance element, means for connecting said master potentiometer to said winding form such that said slider traverses the master potentiometer resistance element as the winding form is rotated, a source of potential connected across the resistance element to be wound and across the master potentiometer resistance element, pick-off means arranged to contact the resistance element to be wound adjacent the point of entry of'the last-named resistance element on the form, means for comparing the ratio of the resistance of the portion of the resistance element already traversed by said pickoif means to the resistance of the portion yet to be traversed withthe' ratio of the resistance of the portion of the master potentiometer already traversed by said slider to the resistance of' the potion yet to be traversed, said means for comparing providing an error signal which is a function of the difference in said ratios, and means for controlling the spacing of adjacent coils of the resistance element being wound as said coils are applied to said form in accordance with said error signal.

12. An apparatus for producing otentiometers, a resistance element whose resistance per unit length may be varied after said element is formed, contact means for traversing the length of a portion of said resistance element, 2. support, means for applying a part of said portion traversed by said contact means rigidly in place on said support so that the distribution of the resistance of said part may not be varied, means for applying a potential across said portion, means for supplying a master reference potential which varies according to a prescribed function, means for detecting the error between the potential at said contact means and said master reference potential and providing a signal which is a function of said error, and means responsive to the error signal for controlling the distribution of the resistance per unit length of said portion which has been traversed by said contact means but which has not yet become rigid.

13. An apparatus for producing a precision resistance device, a resistance element comprising a plurality of successive turns of resistance wire, the spacing of said turns being variable after said element is formed so as to con- References Cited in the file of this patent UNITED STATES PATENTS 2,405,890 Kunz Aug. 13, 1946 2,408,093 Patterson Sept. 24, 1946 2,590,246 Heckman et a1 Mar. 25, 1952 2,620,990 Cary et al Dec. 9, 1952 2,639,864 Hale May 26, 1953 2,643,068 Harris June 23, 1953 2,645,429 Scott et a1 July 14, 1953 2,653,772 Scott Sept. 29, 1953 2,668,670 Van Alen Feb. 9, 1954 2,703,207 Moore Mar. 1, 1955 FOREIGN PATENTS 495,799 Great Britain Nov. 21, 1938